High voltage device fabricated using low-voltage processes

ABSTRACT

A method for fabricating a high-voltage transistor on a semiconductor substrate includes defining and forming shallow trench isolation regions for all of the transistors, defining and forming well regions for all of the transistors, forming a gate oxide layer in the well regions for all of the transistor, forming gates for all of the transistors over the gate oxide layer, implanting a dopant to form lightly-doped drain regions for all of the transistors, the lightly-doped drain regions for at least drains of the high-voltage transistors being spaced apart from an inner edge of the shallow trench isolation regions, forming gate spacers at sides of the gates of all of the transistors, and implanting a dopant to form sources and drains for all of the transistors, the drains of the high-voltage transistors being formed completely surrounded by the lightly-doped drain regions of the high-voltage transistors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/547,336, filed Nov. 19, 2014, which claims the benefit of U.S.Provisional Patent Application No. 61/907,235 for “High Voltage DeviceFabricated Using Low-Voltage Processes” filed Nov. 21, 2013, thecontents of which are incorporated in this disclosure by reference intheir entirety.

BACKGROUND OF THE INVENTION

Programming Silicon-Oxide-Nitride-Oxide-Silicon (SONOS) nonvolatilememory requires medium high or high programming voltages in relation toother voltages used on the device. The devices used to provide theseprogramming voltages should have sufficiently high junction breakdownvoltage and are usually fabricated using gate oxide layers thicker thanstandard I/O devices to increase gate breakdown voltage. Incorporatingthe formation of these devices into existing complementarymetal-oxide-semiconductor (CMOS) fabrication processes usually involvesadditional masks and process steps that are not part of conventionalCMOS fabrication processes.

More particularly, to achieve sufficient gate and junction breakdownvoltages, existing high (larger than 10V) or medium high (5 to 10V)voltage devices use customized doping profiles, especially at the edgesof shallow trench isolation (STI) regions defining the active areas ofthese devices, as well as the aforementioned thicker gate, all of whichcontributes to lower yield.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a high voltage (5 to10V) transistor is disclosed that can be fabricated using conventionalCMOS processes, without the need to provide additional masking and otherprocess steps. The transistor includes lightly-doped regions surroundingat least the drain region and optionally the source region. Thelightly-doped regions extend outwardly towards edges of an active areadefined by inner edges of shallow trench isolation (STI), however outeredges of the lightly-doped regions are spaced apart from the inner edgesof the shallow trench isolation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of an illustrative layout of a high-voltagetransistor in accordance with one aspect of the present invention.

FIG. 2 is a cross sectional view of the layout of a high-voltagetransistor of FIG. 1 taken along lines 2-2 in a direction across thewidth of the channel.

FIG. 3 is a cross sectional view of the layout of a high-voltagetransistor of FIG. 1 taken along lines 3-3 near the drain edge of thegate in a direction along the drain edge of the channel.

FIG. 4 is a top view of an illustrative layout of a high-voltagetransistor in accordance with another aspect of the present invention.

FIG. 5 is a cross sectional view of the layout of a high-voltagetransistor of FIG. 4 taken along lines 5-5 in a direction across thewidth of the channel.

FIG. 6 is a cross sectional view of the layout of a high-voltagetransistor of FIG. 4 taken along lines 6-6 near the drain edge of thegate in a direction along the drain edge of the channel.

FIG. 7 is a flow chart showing an exemplary fabrication process for thehigh-voltage transistors of the present invention.

DESCRIPTION OF THE INVENTION

Persons of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and not in anyway limiting. Other embodiments of the invention will readily suggestthemselves to such skilled persons. The below embodiments areparticularly described in relation to an n-channel device formed inP-well, it being understood that a p-channel device formed in an N-wellis similarly formed.

Referring to FIGS. 1 through 3, top and cross sectional views show anillustrative layout of a high-voltage transistor 10 fabricated inaccordance with one aspect of the present invention in which the drainside of the transistor is pulled back from the diffusion edge. FIG. 1 isa top view, FIG. 2 is a cross sectional view taken along lines 2-2 ofFIG. 1 in a direction across the width of the channel, and FIG. 3 is across sectional view taken along lines 3-3 of FIG. 1 near the drain edgeof the gate in a direction along the drain edge of the channel.

The active area of the high-voltage transistor 10 is a p-well region 12that lies within shallow trench isolation region 14. N+ source region 16and N+ drain region 18 are formed in p-well 12. Lightly-doped drain(LDD) regions 20 and 22 surround source and drain regions 16 and 18,respectively, and define a channel in between the source and drain. Gate24 is disposed above and insulated from the substrate over the channel.Spacers 26 are formed on the side edges of the gate to facilitateformation of the LDD regions 20 and 22 by blocking the highersource/drain implant at the gate edges as is known in the art. In atypical embodiment, the LDD doping level is between about 5e16 and 5e17cm−3, and the source/drain implant doping is between about 1e19 and 1e20cm−3. The spacers are shown in FIG. 2 and are not indicated in FIG. 1 toavoid overcomplicating the drawing figure.

As shown in FIG. 2, the edge of the LDD region 22 in the p-welldiffusion 12 extending outwardly (towards the right side of FIG. 2) fromthe drain 18 of transistor 10 is spaced inwardly from the inner edge ofthe STI region in the present invention as shown at reference numeral 28a. In a typical embodiment, the drain LDD region 22 surrounding thedrain region 18 is spaced inwardly from inner edge of the STI region 14by between about 100 nm and 500 nm. The diffusion edge is where devicesusually break down first due to the presence of the highest electricfields in these regions. This inward spacing is also performed at theedges of the channel width, i.e., near the end edges of the gate 24 asindicated by arrows 28 b in FIG. 1 and by reference numerals 28 b inFIG. 3. As may be seen from an examination of both FIG. 1 and FIG. 3,the ends of the gate 24 extend beyond the outer edges of the LDD regionsat 28 b and even into the area above the STI boundary of the transistoractive area.

Pulling back the outer portions of the LDD region 22 changes thepotential contour around the drain 18 and significantly lowers theelectric field at the edge of the STI region 14. With this, drainjunction breakdown voltage increases significantly, and will easily meeta voltage breakdown requirement of about 8 volts or higher. This inwardspacing is important on the drain side of the devices where the highestvoltages will be found during normal device operation. This decreasesmask symmetry somewhat. An individual designer will weigh this tradeoffat design time. While persons of ordinary skill in the art will realizethat what is a “high voltage” will scale with shrinking device sizes,the principles of the present invention will still be valid.

To further improve junction breakdown, a salicide block layer 30 isintroduced at least at the drain side so that only silicon in thevicinity of the source, drain, and gate contacts is salicided (i.e.,converted to a metal salicide). Persons of ordinary skill in the artwill appreciate that, for simplicity, the top view of FIG. 1 shows asingle contact to each of the source and drain regions 16 and 18, andthat multiple contacts may be employed in an actual integrated circuitfabricated according to the teachings of the present invention. Metalsalicide regions 32 are shown in FIG. 2 in contact apertures at theupper surfaces of the source 16, drain 18, and gate regions 24 as isknown in the art. Because the outer edges of the diffusions (p-well 12)are covered by the salicide block layer 30 that extends over the inneredges of STI regions 12, they have not been converted to metal salicide.Consequently, they have a lower electric field and leakage, as well asmuch reduced joule heat generated at the drain corners. The robustnessof the transistor is thereby much improved.

Referring also to FIGS. 4 through 6, top and cross sectional views showan illustrative layout of a high-voltage transistor 40 fabricated inaccordance with another aspect of the present invention in which boththe drain side and the source side of the transistor are pulled backfrom the diffusion edge. FIG. 4 is a top view, FIG. 5 is a crosssectional view taken along lines 5-5 of FIG. 4 in a direction across thewidth of the channel, and FIG. 6 is a cross sectional view taken alonglines 6-6 of FIG. 4 near the drain side of the gate in a direction alongthe drain edge of the channel.

Transistor 40 of FIGS. 4 through 6 is similar to transistor 10 of FIGS.1 through 3. Elements of transistor 40 that are the same as elements oftransistor 10 of FIGS. 1 through 3 are designated by the same referencenumerals used to identify corresponding elements in FIGS. 4 through 6.

The active area of the high-voltage transistor 40 is a p-well region 12that lies within shallow trench isolation region 14. N+ source region 16and N+ drain region 18 are formed in p-well 12. Lightly-doped drain(LDD) regions 20 and 22 surround source and drain regions 16 and 18 anddefine a channel in between the source and drain. Gate 24 is disposedabove and insulated from the substrate over the channel. Spacers 26 areformed on the edges of the gate to facilitate formation of the LDDregions 20 and 22 by blocking the higher source/drain implant at thegate edges as is known in the art. In an exemplary embodiment, the LDDdoping level is between about 5e16 and 5e17 cm−3, and the source/drainimplant doping is between about 1e19 and 1e20 cm−3. The spacers areshown in FIG. 5 and are not indicated in FIG. 4 to avoidovercomplicating the drawing figure.

As shown in FIGS. 5 and 6, the edges of the LDD regions 20 and 22 in thep-well diffusion 12 that extend outwardly towards the STI region 14 fromboth the drain 18 (towards the right side of FIG. 5) and the source 16(to the left side of FIG. 5) of transistor 40, respectively, are spacedinwardly from the inner edges of the STI regions 14 in the presentinvention. In an exemplary embodiment, the LDD regions 20 and 22 arespaced inwardly from the inner edge of the STI 14 by between about 100nm and 500 nm. In the embodiment of the invention depicted in FIGS. 4through 6, this inward spacing is performed at both the edge of the LDDregion 22 at the drain side and the source side of the transistor 10 asindicated at reference numerals 28 a, as illustrated in FIG. 5, andalso, as in the embodiment depicted in FIGS. 1 through 3, at the edgesof the channel near the ends of the gate 24 as indicated by arrows 28 bin FIG. 4 and reference numeral 28 b in FIG. 6. As may be seen from anexamination of both FIG. 4 and FIG. 6, the ends of the gate 24 extendbeyond the outer edges of the outer LDD regions at 28 b and even intothe area above the STI boundary of the transistor active area.

Pulling back drain n+ implant and the outer portions of the LDD regionschanges the potential contour around the drain and significantly lowersthe electric field at the edge of the STI region 12. With this, drainjunction breakdown voltage increases significantly, and will easily meetmedium high voltage requirement of about 9 volts. As noted, this inwardspacing is important on the drain side of the devices where the highestvoltages will be found during normal device operation, but in thisembodiment of the present invention, the inward spacing is also providedat the source side as shown in FIGS. 4 through 6. This allows moresymmetrical masks to be used but the pullback of the LDD region at thesource 16 will increase the source impedance somewhat. An individualdesigner will weigh these tradeoffs at design time.

To further improve junction breakdown, a salicide block layer 30 isintroduced at least at the drain side so that only silicon in thevicinity of the contacts, including the gate contact 32, is salicided(i.e., converted to a metal salicide). As shown in FIG. 5, the salicideblock layer 30 may be similarly introduced at the source side forsymmetry. Persons of ordinary skill in the art will appreciate that, forsimplicity, the top view of FIG. 4 shows a single contact to each of thesource and drain regions 16 and 18, and that multiple contacts may beemployed in an actual integrated circuit fabricated according to theteachings of the present invention. Metal salicide regions 32 are shownin FIG. 5 in contact apertures at the upper surfaces of the source,drain, and gate regions as is known in the art. Because the outer edgesof the diffusions (p-well 12) are covered by the salicide block layer 30that extends over the inner edges of STI regions 12, they have not beenconverted to metal salicide. Consequently, they have a lower electricfield and leakage, as well as much reduced joule heat generated at thedrain corners. The robustness of the transistor is thereby muchimproved.

The high-voltage transistors of the present invention can be fabricatedusing a conventional low voltage logic CMOS process flow. Referring nowto FIG. 7, an exemplary process 40 for fabricating the high-voltagetransistors of the present invention is shown. The process starts artreference numeral 42. At reference numeral 44, STI regions are definedand formed using conventional photolithography and etching steps. Next,the bottoms of the trenches are doped with channel-stop implants usingconventional implanting steps. The trenches are then filled with adielectric material using conventional deposition techniques.

At reference numeral 46, the p-wells and n-wells for all of the devicesare formed using conventional lithography and dopant diffusiontechniques. Persons of ordinary skill in the art will appreciate that,in conventional CMOS processes, the well formation steps may beperformed either before or after the STI formation steps.

Next, as shown at reference numeral 48, a gate oxide layer for all ofthe transistor devices is grown or deposited using conventionaltechniques. At reference numeral 50, a layer of polysilicon is depositedand defined to form the gates for all of the transistor devices on theintegrated circuit, also using known techniques.

At reference numeral 52, the LDD regions for all devices on theintegrated circuit are formed. A mask for the lightly-doped-drain (LDD)regions is applied using conventional photolithography steps. The LDDmask is already used in a conventional CMOS process. The mask geometryis altered to accommodate the features of the invention that are shownin FIG. 16, particularly the pull back from the formed STI region of thedrain LDD, and optionally the pull back from the formed STI region ofthe source LDD. The LDD regions are then implanted using conventionalion implantation steps.

After the LDD regions have been formed, at reference numeral 54 gatespacers are formed at the gate edges as is known in the art. Atreference numeral 56 an N+ mask is then applied using conventionalphotolithography steps. The N+ mask is already used in a conventionalCMOS process to form all of the n-channel transistors in the circuit.The source and drain regions for all devices are then implanted usingconventional implantation steps.

At reference numeral 58, a salicide block layer is then defined andformed using lithography, deposition, and etching steps as is known inthe art. This process sequence is already present in a conventional CMOSprocess employing salicided contacts to form a salicide block layerconfigured to pull back salicide regions from gate edges in I/Otransistors to provide electrostatic discharge protection. In accordancewith one aspect of the present invention, the existing mask for thisprocess is modified to add the features of the salicide block layer ofthe present invention so that the oxide etch process for the salicideapertures incorporates the geometry of the salicide block layer taughtherein, in particular that only silicon in the N+ regions and gateregions is salicided, and that salicide is not formed at the edges ofthe active region at the inner edges of the STI regions.

At reference numeral 60, the metal layer for the salicide is thendeposited and rapidly annealed to form salicide regions in the aperturesof the salicide mask as is known in the art. The portions of the metallayer overlaying the salicide mask that have not been converted to metalsalicides are then removed as is known in the art, for example by aselective metal etching step.

At reference numeral 62, the normal back-end process steps are thenperformed, including depositing dielectrics, formation and definition ofone or more metal interconnect layers and connection vias, and devicepassivation. The process ends at reference numeral 64.

From the above process description, persons of ordinary skill in the artwill readily appreciate that the high-voltage transistor of the presentinvention can be fabricated without altering existing CMOS fabricationprocesses. The processes accommodate the high-voltage transistors of thepresent invention by altering the geometry of several of the masks usedin the already existing mask set for the process in order to accommodatethe geometric features of the invention disclosed herein.

The present invention provides a significantly simpler fabricationprocess as compared to the conventional method, and a significant totalfootprint reduction as compared to other possible solutions such assource/drain extension MOS devices. The new device is fully compatiblewith existing process, and readily scalable in channel width and length,which is critical for efficient circuit design.

While embodiments and applications of this invention have been shown anddescribed, it would be apparent to those skilled in the art that manymore modifications than mentioned above are possible without departingfrom the inventive concepts herein. The invention, therefore, is not tobe restricted except in the spirit of the appended claims.

1-13. (canceled)
 14. A method for fabricating a high-voltage transistoron a semiconductor substrate comprising: defining and forming shallowtrench isolation regions in the semiconductor substrate for all of thetransistors to be formed in the integrated circuit; defining and formingwell regions in the semiconductor substrate for all of the transistorsto be formed in the integrated circuit; forming a gate oxide layer inthe well regions for all of the transistors to be formed in theintegrated circuit; forming gates for all of the transistors to beformed in the integrated circuit over the gate oxide layer; implanting adopant to form lightly-doped-drain regions for all of the transistors tobe formed in the integrated circuit, the lightly-doped drain regions forat least drains of the high-voltage transistors being spaced apart froman inner edge of the shallow trench isolation regions where drains ofthe high-voltage transistors are to be formed; forming gate spacers atsides of the gates of all of the transistors to be formed in theintegrated circuit; and implanting a dopant to form sources and drainsfor all of the transistors to be formed in the integrated circuit, thedrains of the high-voltage transistors being formed completelysurrounded by the lightly-doped drain regions of the high-voltagetransistors.
 15. The method of claim 14, further comprising: forming asalicide block layer for all of the transistors to be formed on theintegrated circuit, the salicide block layer on the high-voltagetransistors having a geometry to allow salicide to be formed only ondrain regions and on gate regions of the high-voltage transistors; andforming a metal salicide layer for all of the transistors to be formedin the integrated circuit.
 16. The method of claim 14 wherein implantinga dopant to form lightly-doped-drain regions for all of the transistorsto be formed in the integrated circuit includes implanting a dopant toform lightly-doped-drain regions for all of the transistors to be formedin the integrated circuit, the lightly-doped drain regions for thedrains and sources of the high-voltage transistors being spaced apartfrom all inner edges of the shallow trench isolation regions.
 17. Themethod of claim 15 wherein forming the salicide block layer for all ofthe transistors to be formed on the integrated circuit includes formingthe salicide block layer on the high-voltage transistors to extend overinner edges of the shallow trench isolation regions.
 18. The method ofclaim 15 wherein forming the salicide block layer for all of thetransistors to be formed on the integrated circuit further includesforming the salicide block layer on the high-voltage transistors so thatsilicon in the N+ source regions of the high-voltage transistors issalicided.
 19. The method of claim 14 wherein implanting a dopant toform sources and drains for all of the transistors to be formed in theintegrated circuit comprises implanting a dopant to a level of betweenabout 1e19 and about 1e20 cm⁻³.
 20. The method of claim 14 whereinimplanting a dopant to form lightly-doped-drain regions for all of thetransistors to be formed in the integrated circuit comprises implantinga dopant to a level of between about 5e16 and about 5e17 cm⁻³.
 21. Themethod of claim 14 wherein implanting a dopant to formlightly-doped-drain regions for all of the transistors to be formed inthe integrated circuit, the lightly-doped drain regions for at leastdrains of the high-voltage transistors being spaced apart from an inneredge of the shallow trench isolation regions where drains of thehigh-voltage transistors are to be formed, comprises spacing thelightly-doped drain regions for at least drains of the high-voltagetransistors apart from an inner edge of the shallow trench isolationregions by between about 100 nm and about 500 nm.
 22. The method ofclaim 16 wherein implanting a dopant to form lightly-doped-drain regionsfor all of the transistors to be formed in the integrated circuit, thelightly-doped drain regions for the drains and sources of thehigh-voltage transistors being spaced apart from all inner edges of theshallow trench isolation regions comprises spacing the lightly-dopeddrain regions for at least drains of the high-voltage transistors apartfrom all inner edge of the shallow trench isolation regions by betweenabout 100 nm and about 500 nm.